Presently, hard disk drives are one of the more commonly used forms of electronic data storage devices. Hard disk drives contain one or more magnetic disks (“platters”) which serve as the storage media. Each platter includes a plurality of concentric circular tracks on its surfaces, and each platter is divided into geometrical sectors. The intersections of the geometrical sectors and the concentric circular tracks define data sectors, which typically consist of 512 bytes of data storage space each. Magnetic heads move into position over the appropriate tracks (i.e., seeking) and, as the platters rotate beneath, the magnetic heads write data to one or more sectors on those tracks as a series of magnetic polarity transitions. The magnetic heads can also read stored data by detecting these magnetic polarity transitions (or the absence thereof).
Hard disk drives typically include a disk buffer, which is an embedded memory that helps increase performance. For example, when a request is received for one or more sectors of data, there is a reasonable likelihood that subsequent requests will be made for data in the sectors located before and after the requested data on the same track. Storing that data in the disk buffer therefore helps increase performance, as the request and subsequent requests for the data are fulfilled from the disk buffer rather than requiring additional seeking and reads from the platters.
A storage controller manages multiple hard disk drives and can present the hard disk drives to a computer as one or more logical volumes. For example, a storage controller can receive application requests to write data to a particular volume and, based on the geometry of the platters (e.g., track density, sector size, etc.), the storage controller can instruct the appropriate hard disk drive to write the data in the appropriate sectors.
Storage controllers typically have the ability to replicate volumes in the background while still fulfilling input and output (I/O) requests made by applications. A typical replication process involves sequentially reading sectors from a first volume (i.e., on a first hard disk drive) and writing those sectors to a second volume (i.e., on a second hard disk drive). Such replication processes can involve considerable head movement between different tracks of the hard disk drive, resulting in seek delays that can affect the performance of both the replication process and the fulfillment of application I/O requests.